1. Field of the Invention
This invention relates to a liquid crystal device for a liquid crystal display device, a liquid crystal-optical shutter, etc., particularly a liquid crystal device by use of a ferroelectric liquid crystal, and more particularly to a liquid crystal device improved in display characteristics achieved by improvement of the initial aligning state of the liquid crystal molecules.
2. Related Background Art
A display device of the type which controls transmitted light in combination with a polarizing device by utilizing the refractive index anisotropy of ferroelectric liquid crystal molecules has been proposed by Clark and Lagerwall (Japanese Laid-open Patent Publication No. 107216/1981, U.S. Pat. No. 4,367,924). This ferroelectric liquid crystal has generally the chiral smectic C phase (SmC*) or H phase (SmH*) and, in this state, has the property of taking either one of the first optically stable state and the second optically stable state responding to the electrical field applied and maintaining such state in absence of application of electrical field, namely bistability, and it responds rapidly to the change in electrical field. Thus, it is expected to be useful for high-speed and memory-type display devices.
For an optical modulating device employing a liquid crystal exhibiting such bistability to give desirable driving characteristics, it is required that the liquid crystal held between a pair of parallel substrates should be in a state of a molecular arrangement so that transition between the above two stable states may effectively occur independently of the state of an applied electrical field. For example, as to a ferroelectric liquid crystal in a SmC* or SmH* phase, it is necessary to form a domain (monodomain) in which the liquid crystal molecule phase in an SmC* or SmH* phase is arranged perpendicularly to the substrate surface, and therefore the liquid crystal molecule axes are arranged in substantially parallel to the substrate surface.
Whereas, for aligning ferroelectric liquid crystal, there has been a method by use of an alignment control film treated for uniaxial aligning by way of rubbing treatment or oblique vapor deposition treatment.
Most of the conventional alignment methods have been used for ferroelectric liquid crystal having a helical structure exhibiting no bistability. For example, the alignment method disclosed in Japanese Laid-open Patent Publication No. 230635/1985 controls alignment of a ferroelectric liquid crystal by employing a polyimide film treated for rubbing in a state of a helical structure exhibiting no bistability.
However, when the alignment control film of the prior art as described above is applied to alignment control of a ferroelectric liquid crystal with a non-helical structure exhibiting bistability reported by Clark and Lagerwall, the problems as described below were involved.
The present inventors have found that the tilt angle (the angle shown in FIG. 3 as described below is smaller in the ferroelectric liquid crystal with a non-helical structure obtained by alignment with the alignment control film of the prior art as compared with the tilt angle .THETA. (which corresponds to 1/2 value of the apex angle of the cone shown in FIG. 2 as described below) in a ferroelectric liquid crystal having a helical structure. Particularly, the tilt angle .eta. in the ferroelectric liquid crystal with a non-helical structure obtained by alignment with the alignment control film of the prior art was found to be generally about 10.degree., and the transmittance therein was at most about 3 to 5%.
Thus, according to Clark and Lagerwall, the tilt angle in the ferroelectric liquid crystal of a non-helical structure realizing bistability may be expected to have the same angle a the tilt angle in the ferroelectric liquid crystal of a helical structure, but in fact the tilt angle .theta. in a non-helical structure is smaller than the tilt angle .THETA. in the helical structure. Besides, the cause for the smaller tilt angle .theta. in the non-helical structure .THETA. in the helical structure has been found to be attributable to the twisted arrangement of the liquid crystal molecules in the non-helical structure. In other words, in the ferroelectric liquid crystal having a non-helical structure, the liquid crystal molecules are arranged twistedly at a twist angle .delta. successively from the axis 42 of the liquid crystal molecule adjacent to the upper substrate to the axis 43 of the liquid crystal molecule adjacent to the lower substrate (in the direction of twisted arrangement 44) relative to the normal line of the substrate as shown in FIG. 4, and this is a cause for the tilt angle .theta. in a non-helical structure being smaller than the tilt angle .THETA. in the helical structure.
In the Figure, 41 represents uniaxial aligning axis obtained by the rubbing treatment or the oblique vapor deposition treatment on the upper and lower substrates.
Whereas, in a liquid crystal device utilizing birefringence of a liquid crystal, the transmittance through crossed Nicols may be represented as follows: ##EQU1## wherein I.sub.O is incident light intensity, I is transmitted light intensity, .theta. is a tilt angle, .DELTA.n is a refractive index anisotropy, d is a film thickness of the liquid crystal layer and .lambda. is a wavelength of incident light.
The tilt angle 0 in a non-helical structure as described above appears as the angle in the average molecular axis direction of the liquid crystal molecules with twisted arrangement under the first and the second alignment states. According to the above formula, transmittance reaches the maximum at the tilt angle of 22.5.degree., but the tilt angle .theta. in a non-helical structure realizing bistability is at most about 10.degree., and therefore there is involved the problem that its transmittance is as small as about 3 to 5% and is unsatisfactory when application to a display device is considered.